Wear characteristics of HSLA steel

Wear characteristics of HSLA steel

Wear 252 (2002) 16–25 Wear characteristics of HSLA steel S. Mohan∗ , Ved Prakash, J.P. Pathak Department of Metallurgical Engineering, Institute of T...

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Wear 252 (2002) 16–25

Wear characteristics of HSLA steel S. Mohan∗ , Ved Prakash, J.P. Pathak Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India Received 29 November 2000; received in revised form 12 June 2001; accepted 31 July 2001

Abstract HSLA steel being a promising material in numerous applications, was subjected to wear studies under dry sliding conditions at varying conditions of loads and sliding speeds. Wear debris were extensively studied under optical, stereo- and scanning electron microscopes and also by X-ray diffraction. In order to understand the wear mechanism, wear tracks were examined under optical and scanning electron microscopes. In addition, depth of strain hardened zone below sliding surface was measured under different conditions of loads. This study showed that wear rate initially increased either with increased load or sliding speed and debris generated was a mixture of oxide and metal powders, but after attaining a peak in wear rate, a decreasing trend was observed for load as well as sliding speed studied. This decreasing trend has been attained due to domination of oxidation process as a result of rise in temperature at higher values of loads and sliding speeds. These oxides formed with temperature rise covered the wear tracks and wear rate decreased. Finally, depth of strain hardened zone beneath the sliding surface was observed to increase with load. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Wear rate; Debris; Oxidative; Oxidative–metallic (debris consists of oxide and metal powders); Sliding speed; Depth of strain hardened zone

1. Introduction Wear is a progressive destructive complex phenomena during which deterioration of surfaces occurs in industrial operation and components may fail due to inadequate lubrication, faulty design, excessive handling or mis-operation. It leads to heavy expenditure for maintenance and replacement of industrial plant equipment, causing a significant operating cost to industry, process plant and subsidiary processes, contend with a much bigger wear problem than in the case of machine parts. Though life is much shorter, the reduction in efficiency and loss of system reliability may not be so crucial. However, much of industrial cost of wear incurred may not be so much in the cost of the material removed as in the total effort required to restore original condition [1]. In situ corrections for the efforts of wear are expensive in terms of maintenance, labour and cost of plant or machine shutdown time. A large number of machine components fabricated with various steels fail due to any of reason as mentioned earlier, but main cause is improper material selection. Besides material properties, wear rate can be related to the microstructure which is a result of chemical composition and fabrication. The volume content of different micro-constitutional phases have dominant in-

fluence on the surface life of the machine parts specially being operated under heavy duty service conditions. In view of the above, high strength low alloy (HSLA steel which is widely used as stationary structural components and for mobile equipment such as automobiles, earth moving and mining equipments) has been subjected to wear study for different variables like loads, sliding distance, sliding speeds under dry sliding conditions at room temperature. The critical analysis of debris and surface topography has also been carried out to understand mechanism of wear under different operating conditions. 2. Experimental details The HSLA steel investigated contains (as wt.%) 0.07 C, 1.61 Mn, 0.28 Si, 0.016 P, 0.009 S, 0.62 Cr, 1.13 Ni, 0.03 V, 0.0015 Ti, 0.032 Nb, 0.028 Al and 1.26 Cu, balance Fe. 2.1. Metallographie examination The microstructure of the alloy was studied under Leitz Panphot optical microscope for different phases present. 2.2. Mechanical testing

∗ Corresponding author. Fax: +91-542-368-428. E-mail address: [email protected] (S. Mohan).

Tensile strength was evaluated at room temperature by Instron Testing Machine at a cross-head speed of

0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 8 3 4 - 1

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for mode of damage and nature of distortion at the surface. 2.6. Determination of strain hardened zone below worn surface To determine the strain hardened zone, some of the wear specimens were investigated. The test specimens were sectioned perpendicular to worn surface and micro-hardness measurements were made with respect to distance below the worn surface by Tucon Microhardness Tester.

3. Results 3.1. Microstructure Fig. 1. Photographic view of pin on disc type of wear machine.

0.5 mm/min using 16 mm gauge length and 4.5 mm diameter specimens. Ultimate tensile strength, yield stress, percentage elongation, percentage reduction in area were also determined. Hardness tests of HSLA steel were performed on a Vicker’s hardness tester using a diamond pyramid cone indenter under a load of 10 kg applied for 10 s.

Fig. 2a and b at different magnification show the microstructure of high strength low alloy steel. These figures reveal the presence of acicular bainite providing good ductility to the steel.

2.3. Wear testing Wear testing was carried out on a pin on disc type machine which has a specific feature of direct loading of cylindrical test pin in vertical contact with surface of high carbon and high chromium steel disc hardened to 63 Rc, stimulating the actual service conditions of sliding of parts in relative motions. The photographic view of wear testing machine is shown in Fig. 1. The wear testing machine and test procedure were discussed elsewhere [2]. The tests were carried out at different loads, sliding distances and sliding velocities by weight loss method. The bulk temperature of specimen pin was also measured. Thermocouple tip of a digital temperature indicator was fixed in contact with the specimen pin at 1.5 mm above the mating interface and temperature of the specimen pin was indicated by the digital temperature indicator. 2.4. Examination of wear debris The debris were examined visually for their colour, shape and size under different wear situations. Debris were also examined using magnet for their metallic nature under severe wear conditions. Further, to identify the phases present and to understand wear modes, debris were examined by X-ray analysis as well as under stereo and scanning electron microscopes. 2.5. Examination of worn surface The worn surface of the test pin was examined under optical and JEOL 840A scanning electron microscopes

Fig. 2. Microstructure of HSLA steel showing bainitic structure (a) at low and (b) at high magnifications.

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Table 1 Mechanical properties of HSLA steel Property

Value

Tensile strength (MPa) Yield strength (0.2%) (MPa) Elongation (%) Reduction (%) Hardness (VHN)

894 711 24 55 246

3.2. Mechanical properties The different mechanical properties tested of HSLA steel are tabulated in Table 1. It shows that this steel has very good combination of mechanical properties which is a prime requirement for a good wear resistant material enhancing the life of machine parts and reducing the effective cost. 3.3. Wear characteristics 3.3.1. Effect of sliding distance on wear Fig. 3 shows the weight loss of test specimen with distance for a particular load of 8 kg and sliding speed of 0.5 m/s. It shows high wear loss in initial running in-period followed by steady-state wear. 3.3.2. Effect of load or wear rate Fig. 4a and b show the wear rate against load at different sliding speeds. It is seen that wear rate increases almost linearly upto a certain applied load attains a maxima for all the sliding speeds, but with further increase in load wear rate has a decreasing trend except for lowest sliding speed. It is interesting to note from these curves that peak in wear rate is postponed as sliding speed is increased upto 0.8 m/s, but beyond this speed, peak in wear rate is preponed.

Fig. 4. (a) and (b) The effect of load on wear rate under dry sliding conditions at different sliding speeds.

3.3.3. Effect of sliding speed on wear rate The effect of sliding speed on wear rate at different loads is shown in Fig. 5. It is seen that for all the loads wear rate initially increases with sliding speeds attains a peak in wear and then follows a decreasing trend. Fig. 6 shows a relation between specimen pin temperature and load at different speeds under dry sliding. It is seen that there is rise in specimen pin temperature with load at all the speeds. It indicates that frictional heating has rising trend with increasing load and speed. 3.4. Metallographic evidence Fig. 3. The effect of sliding distance on weight loss under dry sliding conditions.

The surface wear is influenced by sub-surface deformation leading to delamination of material resulting in wear,

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of oxides whereas at same speed when load is increased to 6 kg a composite mode of wear is observed and generated debris is a mixture of oxide and metallic constituents as in Fig. 7b. But when specimen is studied at low load of 3 kg and moderate speed of 0.8 m/s debris still remain oxidative as evident from stereomicrograph in Fig. 8a. Now as loads ranges from 10 to 12 kg for same speed of 0.8 m/s, debris has a mixed mode of wear, i.e. debris consists of oxide and metal powders and broken oxides covering a larger contact area as in Fig. 8b. In a study at a higher load of 10 kg and different sliding speeds of 0.6, 1.0 and 1.2 m/s, it is observed that nature of debris is of composite nature for all the speeds, but as sliding speed is increased from 0.6 to 1.0 m/s level of oxidation is higher as evident in Fig. 9a and b, but as speed is further increased to 1.2 m/s crushed oxides are observed covering a larger area of contact as in Fig. 9c. Fig. 5. The effect of sliding speed on wear rate under dry sliding conditions at different loads.

therefore, it is important to study wear debris and worn-out surfaces. 3.4.1. Wear debris analysis Wear debris has been extensively studied to understand the wear mechanism. It is observed in Fig. 7a that at low sliding speed and low load of 3 kg debris mainly consist

3.4.2. Worn surface topography Worn surface study under optical microscope for 0.6 m/s sliding speed at different loads reveals that at low load of 4 kg scratching of sliding surface is less and oxides are observed, but when load is increased to 8 kg deeper ploughing of surface is observed as in Fig. 10a and b. However, when load is further increased to 12 kg surface is more covered with oxides reducing the ploughing (cf. Fig. 10c). In another study at a load of 5 kg, but at different sliding speeds, it is seen that at low speed of 0.4 m/s scratching of surface is less, but as speed is increased to

Fig. 6. The effect of load on specimen pin temperature under dry sliding at different speeds.

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Fig. 7. The stereo-micrograph showing oxidative mode of wear debris for a sliding speed of 0.2 m/s, but at different loads of (a) 3 kg and (b) 6 kg.

1.0 m/s the ploughing of surface is observed (cf. Fig. 11a and b). As speed is further increased to 1.2 m/s large chunks of oxides are seen covering the whole wear track (cf. Fig. 11c). The extensive study under scanning electron microscope reveals in SEM micrograph (cf. Fig. 12) that at low sliding speed of 0.2 m/s and a load of 3 kg, wear track has minimum deformation with patches of oxides covering it partially. However, at a moderate speed of 0.8 m/s with 8 kg of load surface shows deep grooves and cavities which are resulted by removal of oxides and metal chips from the surface as evident from Fig. 13a. But at same speed and increased load (12 kg) though grooves are deep, but most of the surface is covered with smeared oxides as evident in Fig. 13b. Further, in a study at high sliding speed of 1.2 m/s and load of 2 kg, SEM micrograph in Fig. 14a shows cavity formation and also smearing of oxides. Whereas, at same speed for high load of 12 kg, SEM micrograph shown in Fig. 14b reveals

Fig. 8. The stereo-micrograph showing oxidative and oxidative–metallic mode of wear debris at 0.8 m/s sliding speed, but at loads of (a) 3 kg and (b) 10 kg.

that surface is more or less covered with oxide layer except for a few cavities which are rarely observed. 3.5. Subsurface examination The subsurface examination of the worn surface under scanning electron microscope displays how delamination takes place as proposed by Suh [3]. Fig. 15 under SEM reveals the formation of subsurface cracks much below the surface and strain accumulation just below the surface. It was also observed that with increase in stress the subsurface cracks propagate while joining each other and finally get detached. Further, micro-hardness-depth-profile has been determined for two different loads at same sliding velocity. To find the depth of strain hardened zone micro-hardness measurements have been made with distance from worn surface till values became equal to base value. It was observed that at 5 kg of load strain hardened zone was 100 ␮m, but for 12 kg it was 136 ␮m as evident from Figs. 16 and 17.

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Fig. 9. The stereo-micrographs of debris at a load of 10 kg, but different speeds showing oxidative and oxidative–metallic modes of wear debris (a) 0.6 m/s, (b) 1.0 m/s and (c) 1.2 m/s.

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Fig. 10. The optical micrographs of wear tracks at a speed of 0.6 m/s, but at different loads of (a) 4 kg, (b) 8 kg and (c) 12 kg.

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Fig. 12. The SEM micrograph of the wear tracks at 0.2 m/s sliding speed and load of 3 kg.

Fig. 11. The optical micrographs of wear tracks at a load of 5 kg, but at different sliding speeds of (a) 0.4 m/s, (b) 1.0 m/s and (c) 1.2 m/s. Fig. 13. The SEM micrograph of the wear tracks at 0.8 m/s sliding speed, but a different loads of (a) 8 kg and (b) 12 kg.

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Fig. 14. The SEM micrographs of the wear tracks at sliding speed of 1.2 m/s, but at different loads of (a) 2 kg and (b) 12 kg.

Fig. 16. The micro-hardness-depth-profile of HSLA steel at a load of 5 kg.

Fig. 15. The SEM micrograph of the transverse taper section through wear tracks at a sliding speed of 0.8 m/s and load 7 kg.

4. Discussion The wear behaviour with sliding distance shows higher rate of wear in (cf. Fig. 3) running in period during which load is concentrated only on few junctions, hence, giving more stress. The tangential stress shears these junctions against frictional force whereas in steady-state rate of generation of wear debris is nearly same to rate of removal of the material from mating surface. Several workers while studying different materials have observed same nature of wear with sliding distance [4–7]. While studying wear rate with load (cf. Fig. 4a and b), initially wear rate increases for all the sliding speeds, but after

Fig. 17. The micro-hardness-depth-profile of HSLA steel at a load of 12 kg.

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a certain load wear rate decreases for all the sliding speeds. When load and sliding velocities both are low, a thin film of oxide layer is formed at the interface restricting metal–metal contact. The layer being thin, deforms elastically, but it is not ruptured at low loads. The black powder debris due to sliding is nothing, but oxides generated as seen in Figs. 7a and 8a and it has been further, confirmed by X-ray diffraction. The formation of oxides and its removal as debris is a continuous process in mild wear regime [8–10]. But as the load or sliding speed increases oxidative–metallic wear is observed in Fig. 7b and 8b leading to an increase in wear. So, there is an optimum combination of loads and sliding speeds which leads to transition in wear mode and that causes the postponement or preponement of peak wear seen in Fig. 4a and b. Even at very high loads, a mixed mode of wear is observed. Due to high localised pressure oxides are covering a larger surface area and wear is decreased, but this phenomena is preponed if speed is too high, even at low load there is rise in interfacial temperature causing oxidation which increases contact area and finally reduced wear. Wear tracks under stereo-microscope is again a reconfirmation of the results shown in Fig. 4a and b. These micrographs show that at moderate speed when load is increased, scratching of surface is less and oxides are seen (cf. Fig. 10a), but with increase in load deeper ploughing of surface is observed (cf. Fig. 10b) leading to increase in wear, but as load is very high surface is again covered with oxides reducing the wear (cf. Fig. 10c). It is further evident from SEM studies that at low load of 3 kg and 0.2 m/s sliding speed, wear track surface has minimum deformation and patches of oxides partially covering the surface (cf. Fig. 12), but under moderate sliding speed and at 8 kg load surface with deep grooves and cavities is observed as a result of removal of oxides along with metal chips (cf. Fig. 13a) and further increase in load to 12 kg at same speed shows cavities, but most of the surface gets covered with oxides (cf. Fig. 13b). Further in case of high sliding speed of 1.2 m/s even at low load of 2 kg cavity formation with smearing of oxides is observed (cf. Fig. 14a) whereas when load is increased to 12 kg at same velocity though elongated cavity formation is still observed, but surface is more or less covered with oxides (cf. Fig. 14b). These results are in agreement with other reports [9–15] for different steels. But a typical behaviour at 1.2 m/s sliding speed that it has less wear rate for all loads as compared to a speed of 1.0 m/s which maybe due to high temperature at higher speeds giving rise to oxides [16,17] and resulting in reduction in wear rate. Fig. 5 shows a relation between wear rate and sliding speed where wear rate initially increases because nature of wear changes from oxidative to oxidative–metallic and a peak is attained in wear rate for all loads. Further, it is interesting to note that peak is preponed with increase in load. It is simply because at higher load deformation of the surface takes place even at lower sliding speeds. Now testing under moderate (0.6 m/s) to high sliding speeds of 1.0 and 1.2 m/s, oxidative wear dominates (cf. Fig. 9a–c)

leading to decrease in wear rate. It is further in agreement with optical micrographs of wear tracks in Fig. 10a–c which show that at 0.6 m/s sliding speed scratches are formed and as speed increases to 1.0 m/s deeper ploughing of surface is observed, but further increase in sliding speed gives rise to larger chunks of oxides covering the surface [18–21]. While examining the subsurface under SEM (cf. Fig. 15) strain accumulation and crack formation below the surface are observed. To understand sub-surface phenomena and to correlate it to wear results it is important to study depth of strain hardened zone below worn surface. The metal surface progressively work harden and reaches a maximum hardness depending upon the mode of deformation. In sliding wear, maximum hardness is reached only after a sufficient period of wear at a given load. It was observed that bulk hardness of undeformed structure can not be used for predicting the wear resistance of many materials like amorphous ones inspite of very high hardness. It may be due to lack of work hardening capacity and inhomogeneous strain distribution which favour surface fatigue. In the sliding wear, on the other hand, it has been established that most of the metals are work hardened. The amount of work hardening depends upon the metal, its micro-constituents and also on operating conditions. Sunderrajan [22], systematically studied the depth of plastic deformation beneath the eroded surface. Moore and Douthwait [23] tried to plot strain behaviour below the worn-out surface and showed that it is proportional to abrasive grit size and square root of applied load. The measured wear rate showed the linear relationship with load. In this study micro-hardness-depth-profile measurements have been made at different loads for same sliding velocity of 0.8 m/s. The depth of strain hardened zone was also measured taking actual measurements. The depth of strain hardened zone has been defined as the distance from worn surface where micro-hardness value is same as base value and this study shows that with increase in load depth of strain hardened zone is increased (cf. Figs. 16 and 17).

5. Conclusions It can be concluded from this study that, the wear loss with sliding distance has a initial running-in period followed by steady-state wear. The wear rate initially increases with increase in applied load for all sliding velocities, attains a maxima and then decreases, exhibiting mild oxidative wear at low loads and oxidative–metallic, a mixed mode of wear where a mixture of oxides and metals debris is generated at medium and high loads but again high load regions are dominated by oxidative wear. Wear rate increases initially with sliding velocity for all loads and after attaining a maxima it showed a decreasing trend. The mode of wear are oxidative and oxidative–metallic as in case of load. In case of higher velocity of 1.2 m/s, a reduced wear rate as compared

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to 1.0 m/s speed is observed for all the loads. Wear debris at low loads are brown and black while at higher loads, it becomes black with metallic ash colour of more fragile in nature due to metallic content. At low loads debris consists of FeO and Fe2 O3 whereas at high loads Fe3 C, ␣–Fe and Fe2 O3 are found. Work hardening is observed at subsurface level and depth of hardened zone increases with applied loads. Mild/oxidative wear was characterised by formation of patchy oxide layers at the worn surface with oxidised wear debris. Ploughing by harder asperities and fracture of subsurface hardened zone lead to fine and coarse metallic plate chips in dominant metallic wear region. References [1] [2] [3] [4]

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